Test Data AC1-05

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Ahmed body

Application Challenge 1-05 © copyright ERCOFTAC 2004


Overview of Tests

The first experiment (Exp1) deals with conventional average values such as oil flow patterns, static pressure measured on large times. Thus no information about unsteadiness will be included in the data. In Exp1, conventional wake surveys are performed with 10 hole probes. Drag is measured by strain gauge balance. The contribution of the drag is estimated for each part of the model. (front, slant rear, vertical rear base). The Reynolds number based on the model total length is 4.29 x 106.

The second experiment (Exp2) uses a two-components LDV system. Averages are performed on a high number of samples (40,000) on long time durations, typically 5 minutes.


NAME


GNDPs


PDPs

(problem definition parameters)


MPs (measured parameters)



Re


External velocity


External turbulence level



Slant angle


detailed data


DOAPs

EXP 1 Ahmed original (1984)


4.29 106


60m/s


0.5%


5, 12.5, 25, 30


Pw, Ui


Cx, Flow structure



Re






detailed data


DOAPs

EXP2 Lienhart et al. (2000)


2.78 106


40m/s


0.25%


25, 35


First, second and third moments


Pw, Flow structure

Table EXP-A Summary description of all test cases


Test Case EXP-1

Description of Experiment

See table

Boundary Data

The test section is a ¾ open test section. Only the floor is a solid boundary. The homogeneity and far field conditions are not given.

The incoming turbulence intensity is less than 0.5% for 60 m/s. No details on the incoming turbulence are available.

The size of the nozzle at the entrance of the test section is 3x3 m².

The model is supposed to be smooth. No info is available on the turbulent/laminar nature of the boundary layers on the model. The influence of possible transition can be tested by CFD;

No information is available on the precision of the alignment of the model in the flow, although the symmetry on the visualizations give some confidence on this point. This sensitivity can also be checked by CFD.


No details on the typical time scales are provided. The influence of the unsteadiness of the wake on the averaging process can be estimated through URANS or LES.


Measurement Errors

Flow angle precision ± 0.4°.

Free stream dynamic pressure 1%.

Forces and moments are measured with balances with uncertainty of

± 0.2 N and ± 0.1Nm.


Measured Data

The test data include measurements of:

- Wall pressure

- Visualization of flow patterns on rear (slant) surface

- Wake survey (velocity vector plots, average values) :

- Mean velocity distribution in wake central plane

- Cross flow velocity for several downstream locations

- Drag coefficient: contributions of the pressure and friction drags to the total drag are estimated, as well as the repartition of the pressure drag among the front, slant part and vertical base.


All these data are provided for slant angles j = 5, 12.5, 25 and 30 °.

An additional test is performed by fixing a splitter plate vertically in the wake of the body, in the plane of symmetry.


References

Some salient features of the time-averaged ground vehicle wake, S.R. Ahmed, G. Ramm and G. Faltin, SAE paper series Technical paper 840300, Detroit, 1984


Test Case EXP-2

Description of Experiment

See table

Boundary Data

The test section is a ¾ open test section. Only the floor is a solid boundary. The homogeneity and far field conditions are not given. However the blockage is assumed to be less than 4%.

The incoming turbulence intensity is less than 0.25% for 40 m/s measured by hot wire anemometry 400 mm upstream of the model. The viscosity ratio is about 10.

The models are supposed to be smooth. Transition to turbulence of the boundary layer on the front part is triggered.

No information is available on the accuracy of the alignment of the model in the flow, although the symmetry on the visualizations give some confidence on this point. This sensitivity can also be checked by CFD.

No detail on the typical time scales are provided. The influence of the unsteadiness of the wake on the averaging process can be estimated through URANS or LES.


Measurement Errors

Error on mean velocities is less than 0.005% of local mean in the outer flow. In the wake region the accuracy is assumed to be 1% for mean values and 1.5% for RMS.


Measured Data

LDA measurements of mean velocities: U, V, W, Reynolds stresses Image23.gif Image24.gif Image25.gif Image26.gif Image27.gif and third order moments Image28.gif Image29.gif Image30.gif Image31.gif Image32.gif Image33.gif Image34.gif in some planes for 2 slant angles:

25° slant angle:

planes: Ahmed_25_y=0_global.dat (whole flow); Ahmed_25_y=0_focus.dat (focus on the slant part); y=100; y=180;

y=195; Ahmed_25_y=-195.dat

x=-178; x=-138; x=-88; x=-38; x=0; x=80; x=200; x=500

35° slant angle:

planes: Ahmed_35_y=0_global.dat (whole flow); Ahmed_35_y=0_focus.dat (focus on the slant part); y=100; y=180

x=-88; x=0; x=80; x=200; x=500


Hot wire measurements in the boundary layer in the symmetry plane at different x-location: mean velocities, Reynolds stresses and third moments (only u-w components):

25° slant angle:x=-243, -223, -203, -183, -163, -143, -123, -103,

-83, -63, -43, -23, -3

35° slant angle:x=-243, -223, -203, -183, -163, -143, -123, -103,

-83, -63, -43, -23, -3


Pressure coefficients on the rear of the body:

25° slant angle

35° slant angle


Inlet.dat


References

Flow and Turbulence Structures in the Wake of a Simplified Car Model (Ahmed model),

H. Lienhart, C. Stoots and S. Becker, DGLR Fach Symp. Der AG STAB, Stuttgart University, 15-17 nov. 2000

H. Lienhart and S. Becker, Flow and turbulence structures in the wake of a simplified car model, SAE Paper 2003-01-0656, 2003.


© copyright ERCOFTAC 2004



Contributors: Remi Manceau; Jean-Paul Bonnet - Université de Poitiers

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