======Boundary Layer in a Concave Bend====== =====Experiments by Johnson===== ---- ====Description==== Developing boundary layer submitted to destabilizing concave curvature at \(Re_{\theta} = 1450\) without longitudinal pressure gradient (\(S = \partial p/\partial x \approx 0\)). The test facility is a free surface water channel, shown schematically in , consisting of an asymmetric 4:1 contraction, a 4.88 m long straight development section with dimensions 25 cm (width) by 1.25m (span), a 90 degree constant radius curved wall (\(R = -136\) cm), and a straight recovery section. The channel walls are contoured to minimize pressure gradient in the straight development section and along the concave test surface. The boundary layer is tripped 1 m downstream of the nozzle exit using a 0.476 cm square rod. Spanwise surveys show good spanwise uniformity; specifically, the roll cells which appear in the curved section are not fixed in space, but are randomly distributed in space and time. {{ figs:case023:cas23-exp.png |Experimental arrangement}} LDV measurements of all three velocity-component were made down to \(y^+ \approx 5\); \(u'\) and \(w'\) are at even smaller \(y^+\) at a number of streamwise locations, as indicated in . One set of measurements were taken with no grid-generated turbulence present (without the grid shown in ). For the other sets, a square, biplanar, square bar grid (mesh spacing \(M = 6.35\) cm; ratio of mesh spacing to bar width \(M/w = 4\)) was placed at various locations upstream of the curved wall and used to generate nearly axisymmetric turbulence. In most cases the free-stream turbulence levels were nominally \(u'/U_{pw} = 0.05\). {{ figs:case023:test23.gif |Flow geometry}} ====Measurement Techniques==== Velocities measured using LDV. ====Available Measurements==== For most cases profiles are available at a number of streamwise locations around the bend for * Mean velocity * Reynolds stresses * Triple moments, skewness, flatness [[case023-plots|Sample plots]] of selected quantities are available. The data can be downloaded as compressed archives from the links below, or as individual files. * {{cdata:case023:blcb-allfiles.zip|blcb-allfiles.zip}} * {{cdata:case023:blcb-allfiles.tar.gz|blcb-allfiles.tar.gz}} The file {{cdata:case023:readme.txt|readme.txt}} contains a description of the files and data formats. {{cdata:case023:xdata.dat|xdata.dat}} contains a summary of the channel width as a function of streamwise distance, and values of wall skin friction at measurement locations. ^ Cases Without Turbulence-Generating Grid ^^^ ^ Location ^ Mean velocity & Reynolds stresses ^ Triple moments ^ | \(x=-56\) cm | {{cdata:case023:fn1.dat|fn1.dat}} | {{cdata:case023:fn2.dat|fn2.dat}} | | \(x=35.5\) cm (\(15^o\) station) | {{cdata:case023:cn151.dat|cn151.dat}} | {{cdata:case023:cn152.dat|cn152.dat}} | | \(x=71\) cm (\(30^o\) station) | {{cdata:case023:cn301.dat|cn301.dat}} | {{cdata:case023:cn302.dat|cn302.dat}} | | \(x=106.5\) cm (\(45^o\) station) | {{cdata:case023:cn451.dat|cn451.dat}} | {{cdata:case023:cn452.dat|cn452.dat}} | | \(x=142\) cm (\(60^o\) station) | {{cdata:case023:cn601.dat|cn601.dat}} | {{cdata:case023:cn602.dat|cn602.dat}} | ^ Cases With Turbulence-Generating Grids ^^^ ^ Location ^ Mean velocity & Reynolds stresses ^ Triple moments ^ Notes ^ | \(x=-56\) cm | {{cdata:case023:fg191.dat|fg191.dat}} | | Grid placed at \(x=-177\) cm | | \(x=-56\) cm | {{cdata:case023:fg101.dat|fg101.dat}} | | Grid placed at \(x=-120\) cm | | \(x=35.5\) cm (\(15^o\) station) | {{cdata:case023:cg151.dat|cg151.dat}} | {{cdata:case023:cg152.dat|cg152.dat}} | Grid placed at \(x=-127\) cm | | \(x=71\) cm (\(30^o\) station) | {{cdata:case023:cg301.dat|cg301.dat}} | {{cdata:case023:cg302.dat|cg302.dat}} | Grid placed at \(x=-92\) cm | | \(x=106.5\) cm (\(45^o\) station) | {{cdata:case023:cg451.dat|cg451.dat}} | {{cdata:case023:cg452.dat|cg452.dat}} | Grid placed at \(x=-56\) cm | | \(x=142\) cm (\(60^o\) station) | {{cdata:case023:cg601.dat|cg601.dat}} | {{cdata:case023:cg602.dat|cg602.dat}} | Grid placed at \(x=-32\) cm | ====References==== - Barlow, R.S., Johnston, J.P. (1988). [[https://doi.org/10.1017/S0022112088001545|Structure of a turbulent boundary layer on a concave surface]]. //J. Fluid Mech.//, Vol. 191, pp. 137-176. - Johnson, P.L. (1990). PhD thesis, Stanford University. - Johnson, P.L., Johnston, J.P. (1989). The effects of grid-generated turbulence on a flat and concave turbulent boundary layer. //Report MD-53//, Thermosciences Division, Dept. of Mech. Eng., Stanford University. ---- Indexed data: case : 023 title : Boundary Layer in a Concave Bend author* : Johnson year : 1990 type : EXP flow_tag* : 2d, curvature