This document may be of more than historical interest, because "Attachment D" represents the organizers' judgement of the best/most relevant test cases available to them in 1990. "Attachment E" contains instructions on how to run the various test cases, and these may still be useful suggestions. Some anachronisms have been removed. _____________________________________________________ Attachment D (to project Newsletter 2) Collaborative Testing of Turbulence Models LIST OF PROPOSED TEST CASES 1. INTRODUCTION In addition to the comments from all Collaborators requested in Newsletter no. 1, we have asked some Collaborators to make informal reviews of the data for certain types of flows on which they are experts. The list below is an interpretation (mainly due to Bradshaw) of the replies received. The object is provide a range of test cases which almost all modellers should be able to do, plus some harder ones representing practical problems. If nobody produces calculations for some of the practical cases, that is in itself a piece of evidence about the state of turbulence modelling - which the funding agencies will note! We do not propose to give each new test case a code number - name of author and/or type of flow will be used instead. 2. EXISTING DATA LIBRARIES The basis for the test cases is still the 1980/81 Stanford meeting data library, compiled in packed ASCII format by Brian Cantwell. In some cases we will supply more recent data sets as direct substitutes. The 1.44 MB IBM PC disks, serial nos. D1-D3 contain nearly all 80/81 cases. The exceptions are 0411 (circular cylinder), 0441 (stalled airfoil), 0511 (wing-body junction), 0512 (curved duct) and 8602 (shock/BL interaction): these five are very long files but can be requested from Bradshaw. All files have names equal to the 80/81 cases, with prefix "F", e.g. F0141 for 80/81 case 0141, Samuel/Joubert boundary layer. Files with updated data are on Disk D4 onward and have suffix "A": for example, unpacked and interpolated data for 80/81 case 0141 are on file F0141A. The compressible-flow test cases have been taken from the latest in the review series initiated by Hans Fernholz and John Finley, AGARDograph 315 "A survey of measurements and measuring techniques in rapidly distorted compressible turbulent boundary layers" by H.H. Fernholz, P.J. Finley, J.P. Dussauge and A.J. Smits. We have all the data on disk, but only the most popular test cases are being circulated: the full data set is available from NASA Sci. and Tech. Info. Ctr., phone (301) 621-0204, or from the AGARD National Centers listed on p. 1-3 of AGARDograph 315. Note that AGARD publications may not be officially available outside NATO countries. 3. DATA FORMATS 3.1 Data from the Stanford 1980/81 library tape The 80/81 library is in Brian Cantwell's standard format: all values are given as integers, "intval", in the range 0 to 10000, with maximum and minimum values in separate files, so the real value is "(min)+(intval)*(max-min)/10000": see the extract from the 80/81 Proceedings at the end of this Attachment. The enclosed Disks 1-3 differ from the tape only in that each case occupies only one file: the beginnings and ends of the original tape files, counting serially from "file 1" in the first test case, are labelled in the disk file. Note that internal references to "file" numbers start from no. 1 in each test case. In some cases, the 80/81 files have been superseded by "plain-format" files whose names are those of the 80/81 files, with suffix "A". We have not used Cantwell's normalized format in the new data. In Disk D4, File F0141A contains interpolated initial data and a skin-friction plot for the Samuel-Joubert boundary layer in increasing adverse pressure gradient, 80/81 case 0141: File 8501A contains Papamoschou and Roshko's data for compressible mixing layers, to supersede the correlation in 80/81 case 8501. At least two more updates of 80/81 files will be distributed in "plain-format" style, with instructions for use as test cases. If in doubt, use a file with suffix "A" rather than the original. 3.2 Compressible data from the Fernholz & Finley review. The AGARDograph 315 data are in conventional format, though that on the disks is an improvement over that in the AGARDograph - the double quote used for exponentiation has been replaced by the standard "E". The present organizers have added a few lower-case comments but made no other changes: in particular we have NOT inserted the "skip then read" values discussed in the next section. File names look similar to 80/81 compressible cases but are distinct. The internal notation should be obvious, but note that "Pitot" pressure is the total pressure downstream of a normal shock. 3.3 New test cases For the new test cases, the effort available has allowed us, at most, to edit the data files so that a given line contains either alphabetic information (titles, etc) or numerical values, but not both. That is, modellers will merely have to edit out, or arrange their data-reading programs to ignore, lines that begin with alpha characters, and then use free-format reads for numerical data. We assume that all Collaborators will be able to read free format numbers separated by spaces, e.g. 1729 2.71828 6.6E-23 ........ We are adopting one of two courses. For the shorter data files such as correlations, which are more likely to be edited and plotted than read into the prediction program directly, we are, at most, inserting a "%" sign as the first character of every alphabetic line (this is the "comment" character in TeX and in the PROPLOT plotting utility: modellers who prefer another "ignore this line" sign should be able to edit the "%" sign into something else). For longer files, we are inserting a standard "skip then read" line before each cluster of alphabetic lines followed by data lines, as in the following example.......... 3 5 .... (2I3) skip three lines, then read five In the second century of the Christian era, the Empire of Rome comprehended the fairest part of the earth, and the most civilised portion of mankind. 4004 8008 8080 8086 80286 1 2 .... another skip/read pair 94.305 30.30 ................The modeller's program must be set up to read the two numbers on the "skip/read" line (NS, NR), then read NS lines into a dummy string, then read NR lines, then read the next skip/read pair. 4. TEST CASE DATA - MARCH 1990 DISTRIBUTION To focus modellers' efforts and to give an overview of results as soon as possible, we have nominated a few "Priority" cases for early attention. 4.1 Incompressible-flow "Priority" cases. 4.1.1 Simulation results A major breakthrough since 1980/81 is the availability of simulation data, potentially including all possible turbulence statistics. Data for the NASA Ames simulations of a constant-pressure boundary layer (Spalart, J. Fluid Mech. 187, 61) and a 2D duct ("channel") flow (Kim, Moin and Moser, e.g. J. Fluid Mech. 194, 15) are together in file SIMUL1.DAT on Disk D4. These are nominally easy test cases, but provide a check on individual terms in turbulence models which cannot be measured directly. 4.1.2 80/81 test cases Of the cases in the Stanford 80/81 data library, the Samuel- Joubert boundary layer in increasing adverse pressure gradient (case 0141) tests the ability to handle rapidly- growing flows, which can defeat transport equations for length scale (or time scale or dissipation). The homogeneous turbulence test cases (0371-0376) 80/81 are replaced by fewer and more recent data, except for the classical Comte-Bellot/Corrsin results (0371) for decay of isotropic turbulence. The plane mixing layer (case 0311) may be a difficult one, since its turbulence structure is very different from that of a boundary layer. We need to check which, if any, models can handle it. As in 80/81 calculations should start from a well-developed, but low-Re, turbulent boundary layer and be run long enough for the growth rate to asymptote. The backward-facing step with inclined top wall was a "predictive" test case in 80/81, the measurements being reported by Driver and Seegmiller (AIAA J. 23, 163, 1985). We request that modellers should also compare with the same authors' data for a parallel top wall. 4.3 Compressible flow "Priority" cases. There were comments that the unusually high wall temperature in entry test case (iv) may have contributed to scatter between codes using different properties for air. The Fortran programs for generating the Van Driest skin-friction correlation, circulated with Newsletter no. 1, use the following properties in SI units:- Gas constant 287.2, ratio of specific heats 1.4, Prandtl number 0.72, viscosity 1.458E-6*T**1.5/(T+110.4). Modellers using substantially different properties should check their influence. Stanford 80/81 test cases 8101 and 8201 are for the compressible boundary layer in zero pressure gradient; 8101 requires skin friction on an adiabatic wall over a range of Mach numbers, and 8201 the skin friction on a cooled wall over a range of temperatures at M=5. In view of the scatter in Entry test cases (iii) and (iv) of the present effort, a full re-run of 8101 and 8201 seems desirable - with the additional requirement of Stanton number for case 8201. The two-stream mixing layer, case 8501 (new data in file F8501A), is the only flow which undoubtedly suffers from compressibility effects at non-hypersonic Mach numbers. None of the methods presented at the 1981 Stanford meeting could predict these effects - except by inserting M-dependent coefficients which were not checked for accuracy in wall flows. Despite the remarks about the legitimacy of computerized adjustment of coefficients in Section 3.2 of Newsletter no. 1, we request that, to start with, people should use the exactly the same model for the mixing layer and the boundary layer - i.e. no flow-dependent adjustments as such. This still allows the coefficients to be functions of any Mach number that can be legitimately defined both in the boundary layer and in the mixing layer, e.g. (turbulent k.e.)1/2 / (speed of sound), but the "entry case" boundary- layer computations should of course be repeated if a model is changed to deal with the mixing layer. The boundary layer and mixing layer are nominally self- similar flows, so that initial conditions need not be standardized - but, again, runs should be long enough for self-similarity to be reached. ____________________________________________________________ Attachment E - Instructions for "March 1990" test cases (for a discussion, see Section 4 of Attachment D, above). 1. Contents of disks. Disk D1 contains incompressible data from the Stanford 80/81 Data Library (with a few omissions) Disks D2 and D3 contain compressible data from the Stanford 80/81 Data Library (again with a few omissions). Disk D4 contains new incompressible data, mainly amendments of or replacements for the 80/81 test cases. Disk D5 contains new compressible data, notably four cases from the AGARDograph 315 data library. 2. General instructions "Priority" cases are marked by an asterisk at the start of the subsection: others are at present optional but may be raised to priority level to settle later disputes. It is desirable that all modellers should attempt case 0141 (section 3.1), but apart from this modellers may concentrate on either the incompressible or the compressible cases. To minimise the amount of paper to be circulated, only a very few plots per case are required to begin with. For cases which are expensive or awkward to compute, please keep complete results for each measurement station, in a form that can be plotted at short notice for further discussion. Experimental or simulated results should be always be plotted on the same graph as the computations, and two graphs should be fitted on to each page (A4 users please leave at least 25mm bottom margin!) Each graph should carry the modeller's name, the name of the data file (usually an index number from 80/81 or AGARDograph 315, or an abbreviation of the experimenter's name), and where necessary a description of the sub-case (e.g. "BL" or "DUCT" in file SIMUL1). Velocity profiles should be plotted as U against y, not y against U. Don't use color! 3. Incompressible test cases *3.1 Boundary layer in increasing adverse pressure gradient. This is 80/81 case 0141 (Samuel and Joubert, J. Fluid Mech. 66, 481, 1974). The 80/81 file, F0141 on Disk D1, is just for general background: use file F0141A on Disk D4 for corrected, smoothed and interpolated data. Original turbulence profiles are poorly defined near the boundary-layer edge. Note that pressure data are an accurate numerical integral of dp/dx data and that "dissipation" at the first station has been assumed equal to production. Start at x=1.04m (first station with turbulence measurements: mean profiles in F0141A are interpolated between x=.855 and x=1.16). Continue to x=3.4m (normal pressure gradient may be significant for x > 2.9m). Plot calculated local skin-friction coefficient against x, compared with the first of the three tabulations in F0141A. Also plot max. shear stress at each x, normalized by local edge velocity, compared with values from uv profiles in F0141A. 3.2 Boundary layer and duct simulation data The object in both cases is to plot and compare profiles of the highest-order turbulence quantities your model generates (even if you do not normally print them out), normalized by friction velocity and delta (boundary-layer thickness or half-width of duct). For example, for two-equation models plot the dissipation and the turbulent diffusion of kinetic energy; for stress-equation models, plot the "fast" and "slow" pressure-strain terms in the uv equation and their sum, the turbulent transport ("diffusion") of uv, and the dissipation. We doubt the value of comparing with the individual terms in the dissipation equation, especially at low Reynolds numbers. *3.2.1 Run a constant-pressure boundary layer calculation to match the simulation data (Spalart, J. Fluid Mech. 187, 61), which nominally replace 80/81 test case 0612 (Wieghardt's flow 1400 from the 1968 Stanford meeting). Three sets of profiles are given, all at low Reynolds number (Retheta = 300, 640 and 1400). This creates difficulties about the decay of initial conditions (necessarily at Retheta of at least 300-400), as well as uncertainty about low-Reynolds number effects as such. Therefore we recommend modellers to check that their predicted "wake parameter" asymptotes to, or at least runs parallel to, Coles' data correlation at Re < 10000 (for consistency this check should be done assuming Coles' values of the log law constants, K=0.41 and C=5.0, whether or not the calculation agrees with them). In the table below, "dU" is defined as the maximum deviation of U/u* from the log.law at given Retheta, where u* is the friction velocity: thus dU is (2/K) times the wake parameter. The skin friction coefficient cf is given to help in setting up initial conditions: note that half a cosine curve is a fair approximation to a shear-stress profile. R,th 425 590 855 1150 1450 2050 2650 4150 5650 8600 11500 105cf 590 524 464 426 398 363 340 308 290 269 255 dU 0 .58 1.12 1.46 1.76 2.10 2.34 2.59 2.68 2.68 2.68 One way of allowing initial conditions to decay is to start at Retheta = 300 (failing which, the lowest Reynolds number at which your model allows turbulence to survive) and march forward, increasing the viscosity at each step so that Retheta remains constant, until the results cease to change: then fix the viscosity and run on to Retheta = 600 and 1400. Plot cf and H against Retheta, as well as plotting your highest-order quantities or budget terms to compare with the simulation. *3.2.2 Run a two-dimensional duct calculation at the same pressure gradient (or surface shear stress) as the simulation (Kim, Moin and Moser, e.g. J. Fluid Mech. 194, 15). A run at the same mass flow rate (i.e. using the simulation results as the initial velocity profile of a marching calculation) may be easier to set up and should be adequate, since all results are to be normalized by friction velocity. 3.3 Homogeneous turbulence 3.3.1 Run 80/81 case 0371 (Comte-Bellot and Corrsin, J. Fluid Mech. 25, 657, 1966: see Disk D1). Specification as in 80/81 Proceedings, p. 408: initial conditions are u2=0.306 m2/s2, v2=0.254, w2=0.254, epsilon=15.52m2/s3. Assume standard atmospheric conditions, e.g. kinematic viscosity = 1/66500 m2/s, and run to t=0.35 sec. Plot turbulent kinetic energy against time and deduce decay exponent with free choice of virtual origin (expected value -1.25+0.06). 3.3.2 Calculate two cases of return to isotropy after irrotational plane strain, using data of Le Penven et al. for (Frontiers in Fluid Mechanics, S.J. Davis and J.L. Lumley, eds., Springer 1985, p.1): see Disk D4, files PENVEN.DAT and PENVEN.FOR (for skip and read). The two Le Penven flows, for different signs of stress tensor invariant III, were nominally uniform over the tunnel cross section at exit from the distorting duct, with negligible shear stresses in the chosen axes. Deduce initial dissipation, or other length-scale equivalent used in your model, from given initial Reynolds number, assuming kinematic viscosity = 1/66500 m2/s. Plot the decay trajectory as invariant II against invariant III. *3.3.3 Calculate development of turbulence in a simple shear, using the data of Tavoularis and Karnik (J. Fluid Mech. 204, 457, 1989): see file TAVOU.DAT on Disk D4. The Tavoularis flow is again nominally homogeneous over the cross section, but note that a spatially-developing shear flow cannot remain homogeneous because the x-wise distance that fluid travels in a given time depends on local mean velocity: thus, even if development in time were homogeneous, inhomogeneity over the cross-setion would arise. Perform a genuine spatial-marching computation, with a shear layer thickness of at least 0.3m (the tunnel height). Plot q**2, -uv (equal to K12*q**2), and microscale lambda. Assume that dissipation is 15 nu u2 / (lambda)2, both to get initial dissipation and to deduce lambda from calculations. 3.4 Free shear layers With the exception of the round jet, we propose these as "blind" test cases. The Reynolds numbers and other boundary conditions are approximately those of well-respected experiments in the literature, and later detailed comparisons can be made with these experiments. *3.4.1 Run calculations for the round jet in still air at Reynolds numbers, based on nozzle exit velocity and diameter (d), of 10000 and 100000 (note that product of exit velocity and diameter is [momemtum flux/(.785 rho)]1/2). At each Reynolds number, report best fit to spreading rate in 60