 calcul wall shear stress heat-transfer measur compress flow calcul wall shear stress heat-transfer measur compress flow ludwieg wall shear stress laminar turbul boundari layer incompress flow determin heat-transfer measur surfac instrument investig essenti small local insul heat element embed test surfac size instrument restrict condit thermal boundari layer gener heat element contain local laminar sublay analysi ludweig's theori instrument extend compress flow insul flat plate limit design oper instrument mention assum compress laminar turbul boundari layer flow vicin wall laminar sublay region heat element assumpt permit laminar boundari-layer equat govern equat analysi laminar turbul boundari layer 