Stagnation pressure

Stagnation pressureMain

Community hub

Stagnation pressure

7 pages, 0 posts

0 subscribers

Recent from talks

Be the first to start a discussion here.

Recent from talks

Be the first to start a discussion here.

Contribute something

About hubMembersContent overviewUpdatesRules

Main reference articles

Stagnation pressure

View on Wikipedia

from Wikipedia

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed.
Find sources: "Stagnation pressure" – news · newspapers · books · scholar · JSTOR (April 2009) (Learn how and when to remove this message)

In fluid dynamics, stagnation pressure, also referred to as total pressure, is what the pressure would be if all the kinetic energy of the fluid were to be converted into pressure in a reversible manner.^[1]^{: § 3.2}; it is defined as the sum of the free-stream static pressure and the free-stream dynamic pressure.^[2]

The Bernoulli equation applicable to incompressible flow shows that the stagnation pressure is equal to the dynamic pressure and static pressure combined.^[1]^{: § 3.5} In compressible flows, stagnation pressure is also equal to total pressure as well, provided that the fluid entering the stagnation point is brought to rest isentropically.^[1]^{: § 3.12}

Stagnation pressure is sometimes referred to as pitot pressure because the two pressures are equal.

Magnitude

[edit]

The magnitude of stagnation pressure can be derived from Bernoulli equation^[3]^[1]^{: § 3.5} for incompressible flow and no height changes. For any two points 1 and 2:

P_{1}+{\tfrac {1}{2}}\rho v_{1}^{2}=P_{2}+{\tfrac {1}{2}}\rho v_{2}^{2}

The two points of interest are 1) in the freestream flow at relative speed $v$ where the pressure is called the "static" pressure, (for example well away from an airplane moving at speed $v$ ); and 2) at a "stagnation" point where the fluid is at rest with respect to the measuring apparatus (for example at the end of a pitot tube in an airplane).

Then

P_{\text{static}}+{\tfrac {1}{2}}\rho v^{2}=P_{\text{stagnation}}+{\tfrac {1}{2}}\rho (0)^{2}

or^[4]

P_{\text{stagnation}}=P_{\text{static}}+{\tfrac {1}{2}}\rho v^{2}

where:

P_{\text{stagnation}}

is the stagnation pressure

\rho \;

is the fluid density

v

is the speed of fluid

P_{\text{static}}

is the static pressure

So the stagnation pressure is increased over the static pressure, by the amount ${\tfrac {1}{2}}\rho v^{2}$ which is called the "dynamic" or "ram" pressure because it results from fluid motion. In our airplane example, the stagnation pressure would be atmospheric pressure plus the dynamic pressure.

In compressible flow however, the fluid density is higher at the stagnation point than at the static point. Therefore, ${\tfrac {1}{2}}\rho v^{2}$ can't be used for the dynamic pressure. For many purposes in compressible flow, the stagnation enthalpy or stagnation temperature plays a role similar to the stagnation pressure in incompressible flow.^[5]

Compressible flow

[edit]

Stagnation pressure is the static pressure a gas retains when brought to rest isentropically from Mach number M.^[6]

{\frac {p_{t}}{p}}=\left(1+{\frac {\gamma -1}{2}}M^{2}\right)^{\frac {\gamma }{\gamma -1}}\,

or, assuming an isentropic process, the stagnation pressure can be calculated from the ratio of stagnation temperature to static temperature:

{\frac {p_{t}}{p}}=\left({\frac {T_{t}}{T}}\right)^{\frac {\gamma }{\gamma -1}}\,

where:

p_{t}

is the stagnation pressure

p

is the static pressure

T_{t}

is the stagnation temperature

T

is the static temperature

\gamma

is the ratio of specific heats

The above derivation holds only for the case when the gas is assumed to be calorically perfect (specific heats and the ratio of the specific heats $\gamma$ are assumed to be constant with temperature).

Notes

[edit]

^ ^a ^b ^c ^d Clancy, L.J. (1975), Aerodynamics, Pitman Publishing Limited, London. ISBN 0-273-01120-0
^ Stagnation Pressure at Eric Weisstein's World of Physics (Wolfram Research)
^ Equation 4, Bernoulli Equation - The Engineering Toolbox
^ Houghton, E.L and Carpenter P.W. Aerodynamics (2003), Section 2.3.1
^ Clancy, L.J. Aerodynamics, Section 3.12
^ Equations 35,44, Equations, Tables and Charts for Compressible Flow

References

[edit]

L. J. Clancy (1975), Aerodynamics, Pitman Publishing Limited, London. ISBN 0-273-01120-0
Cengel, Boles, "Thermodynamics, an engineering approach, McGraw Hill, ISBN 0-07-254904-1

External links

[edit]

Pitot-Statics and the Standard Atmosphere
F. L. Thompson (1937) The Measurement of Air Speed in Airplanes, NACA Technical note #616, from SpaceAge Control.

Revisions and contributors Edit on Wikipedia Read on Wikipedia

View on Grokipedia

from Grokipedia

Stagnation pressure, also known as total pressure, is the pressure that a flowing fluid would attain if brought isentropically to rest, thereby converting its kinetic energy into pressure energy through a reversible adiabatic process without losses.^[1] This concept arises from the steady flow energy equation and Bernoulli's principle, representing the maximum pressure achievable at a stagnation point where fluid velocity drops to zero.^[2] In incompressible flows, stagnation pressure

p_0

is given by the sum of static pressure

p

and dynamic pressure

\frac{1}{2} \rho v^2

, where

\rho

is fluid density and

v

is flow velocity, as derived from Bernoulli's equation along a streamline.^[3] For compressible flows, such as those in high-speed aerodynamics, the relationship is more complex and involves the Mach number

M

, with the ratio

\frac{p_0}{p} = \left(1 + \frac{\gamma - 1}{2} M^2 \right)^{\frac{\gamma}{\gamma - 1}}

, where

\gamma

is the specific heat ratio of the gas.^[4] This isentropic formulation assumes no entropy increase, making it ideal for reversible processes in nozzles, diffusers, and shock-free flows.^[5] Stagnation pressure is commonly measured using a Pitot tube, a device that faces the oncoming flow and captures the fluid at a stagnation point, allowing the difference between stagnation and static pressures to determine flow velocity.^[6] In aviation and aerospace engineering, it plays a critical role in airspeed indicators, engine performance analysis, and wind tunnel testing, where accurate total pressure data ensures reliable calibration of instruments and prediction of aerodynamic forces.^[7] Deviations from isentropic conditions, such as those caused by shocks or friction, result in total pressure losses, which are key indicators of irreversibility and efficiency in propulsion systems.^[8]

Fundamentals

Definition

Stagnation pressure, also known as total pressure, is the pressure that a moving fluid attains when it is decelerated to rest isentropically, converting its kinetic energy entirely into pressure energy without losses.^[9] This occurs at a stagnation point in the flow field, where the fluid velocity is zero relative to the reference frame, such as the leading edge of an airfoil or the nose of an aircraft.^[7] The concept of stagnation pressure builds on Bernoulli's principle, first articulated by Daniel Bernoulli in his 1738 treatise Hydrodynamica, which related fluid pressure, velocity, and elevation along a streamline.^[10] The term itself emerged in early 20th-century aerodynamics, as engineers applied these ideas to analyze airflow around aircraft during the pioneering era of powered flight.^[11] Physically, stagnation pressure at the stagnation point represents the summation of the static pressure—the ambient pressure within the undisturbed flow—and the dynamic pressure, which quantifies the fluid's kinetic energy per unit volume.^[7] For incompressible flows, this relationship is expressed by the equation

p_0 = p + \frac{1}{2} \rho v^2,

where

p_0

is the stagnation pressure,

p

is the static pressure,

\rho

is the fluid density, and

v

is the flow velocity.^[7] This formulation provides a fundamental measure of the fluid's total energy content in terms of pressure.

Relation to Static and Dynamic Pressure

Stagnation pressure integrates the concepts of static and dynamic pressures, serving as the total pressure in fluid dynamics for flows where these components can be distinctly identified. Static pressure, denoted as

p

, represents the pressure exerted by the fluid on a surface perpendicular to the flow direction, equivalent to the pressure measured in a fluid at rest relative to the measurement point. This is the thermodynamic pressure within the flow field, independent of the fluid's motion.^[3] Dynamic pressure, denoted as

q

, arises from the kinetic energy of the fluid due to its motion and is given by the equation

q = \frac{1}{2} \rho v^2,

where

\rho

is the fluid density and

v

is the flow velocity. It quantifies the pressure equivalent of the fluid's momentum, increasing with the square of the velocity and thus highlighting the energetic contribution of motion to the overall pressure state.^[12] Stagnation pressure, denoted as

p_0

, is the sum of static and dynamic pressures:

p_0 = p + q.

This relation positions stagnation pressure as the maximum pressure attainable when the flow is brought to rest isentropically, converting all kinetic energy into pressure without losses. At a stagnation point, where the velocity component normal to the surface is zero, this total pressure is realized, as in the tip of a Pitot tube where the oncoming flow stagnates, effectively adding the dynamic pressure to the static pressure through a process analogous to vector summation in energy terms.^[3]^[12]^[6]

Incompressible Flow

Derivation

The derivation of stagnation pressure in incompressible flow relies on fundamental conservation principles applied to fluid motion. Consider an inviscid, steady, and incompressible flow along a streamline, where the fluid density

\rho

remains constant and viscous effects are negligible.^[13]^[14] These assumptions simplify the governing equations while capturing the essential pressure-velocity relationship in low-speed flows, such as those below Mach 0.3.^[15] The starting point is Euler's equation, which expresses momentum conservation for an inviscid fluid. Along a streamline, this takes the differential form:

dp = -\rho v \, dv

where

p

is the static pressure,

v

is the flow speed, and

dp

represents the pressure change accompanying a velocity increment

dv

. This equation arises from balancing the pressure gradient force with the inertial deceleration of the fluid as it slows down toward a stagnation point.^[13]^[14] To obtain the stagnation pressure

p_0

, integrate this relation from a point in the free stream (where the static pressure is

p

and velocity is

v

) to the stagnation point (where velocity is zero and pressure is

p_0

\int_p^{p_0} dp = -\rho \int_v^{0} v \, dv.

The left side integrates to

p_0 - p

, while the right side yields

-\rho \left[ \frac{1}{2} v^2 \right]_v^0 = \frac{1}{2} \rho v^2

. Thus,

p_0 - p = \frac{1}{2} \rho v^2,

where

\frac{1}{2} \rho v^2

is the dynamic pressure. This result is the integrated form of Bernoulli's principle for incompressible flow along a streamline.^[13]^[15] The derivation's limitations stem from its assumptions: it neglects viscosity, which can introduce boundary layer effects and energy dissipation in real flows, and presumes constant density, restricting applicability to incompressible regimes. These constraints make the formula ideal for theoretical analysis but require corrections for viscous or high-speed flows.^[14]^[15]

Magnitude and Bernoulli's Equation

In incompressible flow, Bernoulli's equation describes the conservation of mechanical energy along a streamline as

p + \frac{1}{2} \rho v^2 + \rho g h = \constant

, where

p

is static pressure,

\rho

is fluid density,

v

is flow velocity,

g

is gravitational acceleration, and

h

is elevation.^[7] For scenarios at constant height where gravitational effects are negligible (

\Delta h = 0

), the equation simplifies to

p_0 = p + \frac{1}{2} \rho v^2

, with

p_0

representing the stagnation pressure.^[16] The magnitude of stagnation pressure exceeds the static pressure by the dynamic pressure term

\frac{1}{2} \rho v^2

, which quantifies the kinetic energy contribution per unit volume. For example, in a water flow (

\rho \approx 1000 \, \kg/\m^3

) at

v = 10 \, \m/\s

, the stagnation pressure increases by approximately

50{,}000 \, \Pa

above the static pressure.^[17] In engineering applications, stagnation pressure is typically expressed in pascals (Pa) in SI units or pounds per square inch (psi) in imperial systems.^[18] Stagnation pressure embodies the pressure component of the total mechanical energy per unit volume in the flow, corresponding to the state where all kinetic energy converts to pressure without losses.^[5] This total energy perspective underscores its role in assessing flow efficiency and energy balance in incompressible systems.^[7]

Compressible Flow

Isentropic Stagnation Pressure

In isentropic flow, the process of bringing a compressible fluid to rest is assumed to be reversible and adiabatic, maintaining constant entropy throughout. This idealization applies to flows where frictional losses and heat transfer are negligible, such as in well-designed nozzles or diffusers with gradual changes in area. The fluid is typically modeled as an ideal gas with constant specific heats, ensuring that thermodynamic properties follow the perfect gas law.^[4] The derivation of isentropic stagnation pressure begins with the conservation of energy, expressed through the total enthalpy relation for steady flow:

h_0 = h + \frac{v^2}{2}

, where

h_0

is the stagnation enthalpy,

h

is the static enthalpy, and

v

is the flow velocity. For an ideal gas, enthalpy is a function of temperature,

h = c_p T

, leading to the stagnation temperature equation

T_0 = T \left(1 + \frac{\gamma - 1}{2} M^2 \right)

, with

T_0

as stagnation temperature,

T

as static temperature,

\gamma

as the specific heat ratio, and

M = v / a

as the Mach number (

a

being the speed of sound). Under isentropic conditions, the pressure-temperature relation

\frac{T_0}{T} = \left( \frac{p_0}{p} \right)^{\frac{\gamma - 1}{\gamma}}

combines with the above to yield the stagnation pressure formula:

p_0 = p \left(1 + \frac{\gamma - 1}{2} M^2 \right)^{\frac{\gamma}{\gamma - 1}}

This equation quantifies the total pressure

p_0

required to isentropically decelerate the flow to zero velocity, reflecting the conversion of kinetic energy into pressure and internal energy.^[4]^[19] In subsonic flows (

M < 1

), the compressible stagnation pressure exceeds the incompressible prediction

p_0 = p + \frac{1}{2} \rho v^2

because the fluid density increases during deceleration, amplifying the pressure recovery beyond the simple dynamic pressure addition. This density variation, driven by thermodynamic compression, becomes significant at higher Mach numbers; for example, the incompressible approximation introduces errors greater than 1% around M ≈ 0.5. As

M \to 0

, the isentropic formula asymptotically approaches the incompressible limit.^[19] At the critical Mach number

M = 1

, corresponding to sonic conditions, the stagnation pressure achieves a specific ratio

p_0 / p = \left( \frac{\gamma + 1}{2} \right)^{\frac{\gamma}{\gamma - 1}}

, marking the point where flow acceleration reaches the speed of sound in isentropic expansions, such as in nozzle throats. This relation underscores the boundary between subsonic and supersonic regimes under ideal conditions.^[4]

Compressibility Effects

In compressible flows, deviations from the ideal isentropic stagnation pressure arise primarily due to irreversibilities such as shock waves, friction, and heat transfer, leading to entropy generation and a reduction in stagnation pressure.^[8] A prominent example is the loss of stagnation pressure across a normal shock wave in supersonic flow, where the process is adiabatic but irreversible. The stagnation pressure ratio downstream to upstream,

\frac{p_{02}}{p_{01}}

, is given by

\frac{p_{02}}{p_{01}} = \left( \frac{(\gamma + 1) M_1^2}{2 + (\gamma - 1) M_1^2} \right)^{\frac{\gamma}{\gamma - 1}} \left( \frac{\gamma + 1}{2 \gamma M_1^2 - (\gamma - 1)} \right)^{\frac{1}{\gamma - 1}},

where

\gamma

is the specific heat ratio and

M_1

is the upstream Mach number; this ratio is always less than unity for

M_1 > 1

, quantifying the entropy increase.^[8]^[20] In supersonic flows, this post-shock stagnation pressure drop starkly contrasts with subsonic flows, where no such normal shock occurs and stagnation pressure remains conserved under isentropic assumptions; the loss in supersonic cases directly measures the irreversibility and entropy rise.^[8] Friction in adiabatic, constant-area duct flows, modeled by Fanno lines, further reduces stagnation pressure along the flow path, with the ratio

\frac{p_{t2}}{p_{t1}} = \frac{M_1}{M_2} \left( \frac{2 + (\gamma-1)M_2^2}{2 + (\gamma-1)M_1^2} \right)^{\frac{\gamma + 1}{2(\gamma - 1)}}

, where subsonic flows accelerate toward sonic conditions and supersonic flows decelerate, both accompanied by decreasing stagnation pressure.^[21] Similarly, in frictionless flows with heat addition, as described by Rayleigh lines, stagnation pressure decreases due to the thermal effects, with the ratio

\frac{p_{t2}}{p_{t1}} = \frac{(1 + \gamma M_1^2)}{(1 + \gamma M_2^2)} \left( \frac{1 + 0.5(\gamma-1)M_2^2}{1 + 0.5(\gamma-1)M_1^2} \right)^{\frac{\gamma}{\gamma - 1}}

; heat addition drives the flow toward Mach 1, exacerbating the loss.^[22] Modern computational fluid dynamics (CFD) simulations have advanced the analysis of stagnation pressure recovery in supersonic inlets by modeling complex shock interactions and optimizing geometries, such as streamline-traced inlets, to maximize the total pressure ratio while minimizing losses from oblique shocks and boundary layers.^[23]

Measurement and Applications

Measurement Techniques

The primary instrument for directly measuring stagnation pressure is the Pitot tube, which features a stagnation port oriented facing the oncoming flow to capture the total pressure

p_0

by stagnating the fluid at the probe tip.^[6] This design converts the flow's kinetic energy into pressure, allowing a pressure transducer to record the value accurately in subsonic and supersonic regimes when properly calibrated.^[24] A common variant, the Pitot-static tube, integrates both a stagnation port and perpendicular static pressure ports to measure static pressure

p

alongside

p_0

, enabling computation of dynamic pressure as

q = p_0 - p

.^[25] This configuration is widely used in fluid flow diagnostics, where the pressure differential provides insights into flow velocity while relating stagnation pressure to dynamic pressure in a single probe assembly.^[26] Calibration of Pitot tubes typically occurs in controlled environments such as wind tunnels or flow benches to determine the probe's coefficient, which accounts for deviations from ideal behavior.^[27] These calibrations involve comparing probe readings against reference standards, often adjusting for geometric factors like tip shape and installation position to ensure accuracy across operating ranges.^[28] For high-velocity flow field measurement, fiber-optic sensors integrated into aerodynamic probes enable pressure measurements by transmitting pressure-induced optical signals.^[29] Measurement errors in Pitot-based systems arise from blockage effects, where the probe's presence displaces flow and elevates local pressure readings in confined channels, necessitating corrections based on probe-to-duct diameter ratios.^[30] In turbulent flows, spatial averaging over the probe orifice can introduce uncertainties in mean stagnation pressure due to fluctuating velocities, requiring ensemble averaging or turbulence intensity adjustments during data processing.^[31]

Practical Uses in Aerodynamics

In aircraft pitot-static systems, stagnation pressure is measured at the forward-facing port of the pitot tube to determine airspeed, where the calibrated airspeed

v

is derived from the incompressible flow relation

v = \sqrt{\frac{2}{\rho} (p_0 - p)}

v=ρ2(p0−p)

, with $p_0$ as the stagnation pressure, $p$ as the static pressure, and $\rho$ as the air density.^[32] This measurement provides pilots with indicated airspeed, essential for safe navigation and performance calculations in subsonic flight regimes.^[33] In jet engine design, stagnation pressure recovery in the inlet is critical for optimizing thrust, as it captures ram pressure from the freestream to compress incoming air efficiently before combustion, minimizing losses that reduce overall engine efficiency.^[34] High recovery factors, often exceeding 0.95 in modern supersonic inlets, ensure maximal kinetic energy conversion to pressure, directly enhancing specific thrust and fuel economy in high-speed aircraft.^[35] Wind tunnel testing utilizes stagnation pressure ratios to calibrate scaled models, accounting for Reynolds number effects that influence boundary layer behavior and aerodynamic forces across different sizes.^[36] By adjusting the tunnel's stagnation pressure to match prototype conditions, engineers validate lift and drag predictions, ensuring accurate simulation of full-scale performance despite geometric scaling.^[37] In hypersonic flows, such as those encountered by re-entry vehicles, stagnation pressure informs heating rate predictions at the nose cone, where convective heat flux scales with dynamic pressure as $q \propto \rho v^3$ , guiding thermal protection system design to withstand peak temperatures exceeding 2000 K. This application is vital for missions like orbital re-entry, where precise stagnation conditions prevent structural failure during atmospheric deceleration.^[38]

Info Pages

Talk Pages

Special Pages

Stagnation pressure

Recent from talks

Recent from talks

Contribute something

Contribute something

Media Pages

Timelines

Articles

Notes collections

Notes

Notes

Days in Chronicle

Stagnation pressure

Magnitude

Compressible flow

See also

Notes

References

External links