A simulation using the navier-stokes differential equations of the aiflow into a duct at 0.003 m/s (laminar flow). The duct has a small obstruction in the centre that is parallel with the duct walls. The observed spike is mainly due to numerical limitations.

This script, which i originally wrote for scilab, but ported to matlab (porting is really really easy, mainly convert comments % -> // and change the fprintf and input statements)

Matlab was used to generate the image.

  %Matlab script to solve a laminar flow %in a duct problem  %Constants inVel = 0.003; % Inlet Velocity (m/s) fluidVisc = 1e-5; % Fluid's Viscoisity (Pa.s) fluidDen = 1.3; %Fluid's Density (kg/m^3)  MAX_RESID = 1e-5; %uhh. residual units, yeah... deltaTime = 1.5; %seconds? %Kinematic Viscosity fluidKinVisc = fluidVisc/fluidDen;  %Problem dimensions ductLen=5; %m ductWidth=1; %m  %grid resolution gridPerLen = 50; % m^(-1) gridDelta = 1/gridPerLen; XVec = 0:gridDelta:ductLen-gridDelta; YVec = 0:gridDelta:ductWidth-gridDelta;   %Solution grid counts gridXSize = ductLen*gridPerLen; gridYSize = ductWidth*gridPerLen;  %Lay grid out with Y increasing down rows %x decreasing down cols %so subscripting becomes (y,x) (sorry) velX= zeros(gridYSize,gridXSize); velY= zeros(gridYSize,gridXSize); newVelX= zeros(gridYSize,gridXSize); newVelY= zeros(gridYSize,gridXSize);  %Set initial condition  for i =2:gridXSize-1 for j =2:gridYSize-1 velY(j,i)=0; velX(j,i)=inVel; end end  %Set boundary condition on inlet for i=2:gridYSize-1 velX(i,1)=inVel; end  disp(velY(2:gridYSize-1,1));  %Arbitrarily set residual to prevent %early loop termination resid=1+MAX_RESID;  simTime=0;  while(deltaTime)  count=0; while(resid > MAX_RESID && count < 1e2)  count = count +1; for i=2:gridXSize-1 for j=2:gridYSize-1 newVelX(j,i) = velX(j,i) + deltaTime*( fluidKinVisc / (gridDelta.^2) * ... (velX(j,i+1) + velX(j+1,i) - 4*velX(j,i) + velX(j-1,i) + ... velX(j,i-1)) - 1/(2*gridDelta) *( velX(j,i) *(velX(j,i+1) - ... velX(j,i-1)) + velY(j,i)*( velX(j+1,i) - velX(j,i+1))));  newVelY(j,i) = velY(j,i) + deltaTime*( fluidKinVisc / (gridDelta.^2) * ... (velY(j,i+1) + velY(j+1,i) - 4*velY(j,i) + velY(j-1,i) + ... velY(j,i-1)) - 1/(2*gridDelta) *( velY(j,i) *(velY(j,i+1) - ... velY(j,i-1)) + velY(j,i)*( velY(j+1,i) - velY(j,i+1)))); end end  %Copy the data into the front  for i=2:gridXSize - 1 for j = 2:gridYSize-1 velX(j,i) = newVelX(j,i); velY(j,i) = newVelY(j,i); end end  %Set free boundary condition on inlet (dv_x/dx) = dv_y/dx = 0 for i=1:gridYSize velX(i,gridXSize)=velX(i,gridXSize-1); velY(i,gridXSize)=velY(i,gridXSize-1);      end      %y velocity generating vent     for i=floor(2/6*gridXSize):floor(4/6*gridXSize)         velX(floor(gridYSize/2),i) = 0;         velY(floor(gridYSize/2),i-1) = 0;     end      %calculate residual for  %conservation of mass resid=0; for i=2:gridXSize-1 for j=2:gridYSize-1 %mass continuity equation using central difference %approx to differential resid = resid + (velX(j,i+ 1)+velY(j+1,i) - ... (velX(j,i-1) + velX(j-1,i)))^2; end end  resid = resid/(4*(gridDelta.^2))*1/(gridXSize*gridYSize); fprintf('Time %5.3f \t log10Resid : %5.3f\n',simTime,log10(resid));        simTime = simTime + deltaTime; end mesh(XVec,YVec,velX) deltaTime = input('\nnew delta time:'); end %Plot the results mesh(XVec,YVec,velX)