Twenty-Third Symposium on Naval Hydrodynamics (2001) / Chapter Skim
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Unsteady Flow Quantities on Two-Dimensional Foils: Experimental and Numerical Results
Unsteady Flow Quantities on Two-Dimensional Foils: Experimental and Numerical Results
Pages 301-313
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From page 301... ...
Jordan (Jordm 1996) did calculation for kminar flow with Rey olds m mber equal to 25000 Moreo x, empirical models for parietal pres we fluctuations m m acelemted or decelerated flow do not give aco crate results So, up to now, we do not have mmerical tool capable to optimise She bade sections wish respect to hyd odynamic excitation LES is ~ method which calculates flow scales selected by me m of ~ filter, G This one is convoluted to She flow variables which scales arc inter d or are moc?
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From page 302... ...
VSM is not m mtrinsic property of She fluid md hence d pends heavily upon the flow itself Th mbg id viscosity has She sdv mtage of bemg robust, that is to (8) say s dissipative effect inhoduced 6 o gh VSM which tends to tstilise the schemes How ver, such s model is fix fr m capable of describing eve y mechmism of She mrerarions betw en She subg id scales md the lihered ones Th o gh She notion of mbg id viscosity, we assume implicitly that She root of the action of She subg id scales on the filtered scales is energetic in other w ads, w only need the energetic h msfer balance betw en th two kinds of scales to describe She subg id tensor effect However, She energy dsins fiom th lihered scales to the mbg id ones is not She mly mechrni m which d scribes the interaction herseen the two rages of scales (Krsich m 1976)
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From page 303... ...
0.45 m The main characteristics of the foils are given in table 2. Section Plan form Chord Span Maximum Thickness Maximum camber NACA 66 rectangular 0.6 m 0.45 m 6% BA foil Rectangular 0.6 m 0.45 m 7 % 2 % 3.5 % Table 2: Main characteristics of the foils.
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From page 304... ...
For numerical results, we plot both LES results and Navier-Stokes stationary calculation with K-£ model and the same mesh as the one used for LES calculation. The agreement seems to be quite good, especially for the area close to the leading edge and flow velocity of 1 1 m/s.
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From page 305... ...
Near the leading edge, we first notice a laminar attached flow with some lines of axial vorticity. Then, several spots of higher vorticity appear along chord and span.
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From page 306... ...
Then the flow re-attaches and after the closure of the bubble we observe spots of high intensity of transverse vorticity which indicate the location of vortex. On the photograph corresponding to axial vorticity, we also notice, in front of the leading edge detachment, the presence of several lines of higher intensity regularly spaced in span.
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From page 307... ...
, a little separation bubble near the leading edge then a reattachment of the boundary layer and spots of vorticity along the chord. Numerical pressure signal comparison: Let us go through the time dependent description of the flow for different given control points.
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From page 308... ...
For BA foil at 7 degrees, the velocity map and flow vorticity both indicate the presence of a separation bubble near the leading edge. So, figure 17 shows the fluctuating pressure at three control points (x/c=0.03; x/c=0.07 and x/c=0.14.
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From page 309... ...
With LES calculation, we are able to know the exact low frequency level. Then, since the model, at the moment, filters to early, we can use this low frequency model to calibrate empirical models.
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From page 310... ...
BA foil at 7 degrees; figure 22) photographs show sheet cavitation related to separation bubble which was observed on numerical photographs and with numerical pressure fluctuation versus time.
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From page 311... ...
The accuracy of the numerical model applied to real flows (with adverse pressure gradient) is better than that of empirical models for the prediction of pressure fluctuation spectrum.
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From page 312... ...
23, pp 601-639,1991 Sagaut P 'Simohtions m mbriqu s d6coulements d coile vec des modeies d s u-maille", Ph f esis, um~versite de Paris , J in 1995 Smagonnsky J., "General cl uhtion experime t wifh fhe primitive equstions ~ the basic experiment" Mo thlyWectherReview,vol 91,pp99-165, 1963
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From page 313... ...
As for validation of unsteady calculations the only results shown concern She pow r spectra of pressme fluctuations But there mything beyond 100 H seems to be numerical noise it is necessary to compare the unsteady pressme fluctuations duectly to measurement to seeiftheamplit de mdthefrequencycmbe predated AUTHOR'S REPLY The cell si e in She boundary layer is Opt ally I I mm following the chord Ed I I mm following She sp m The fu st grid point is located et y+= I Ed w mesh in altitude up to y+=100200 using c geometri 91 ratio of 1 5 with hexched ons 200000 grid pomts are located in this zone (16% of the total amount of the cells mmmber m She domain ) The time step used for the simulation is 0 0005 seconds Ed it is not o priori related to the resolution limits of 1 00h No simulation ht.
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