BrownianMotionForce: Removed the spurious additional 'eta'

Resolves bug-report http://bugs.openfoam.org/view.php?id=2153

src/lagrangian/turbulence/submodels/Thermodynamic/ParticleForces/BrownianMotion/BrownianMotionForce.C

@@ -171,7 +171,6 @@ Foam::forceSuSp Foam::BrownianMotionForce<CloudType>::calcCoupled
     const scalar dp = p.d();
     const scalar Tc = p.Tc();
 
-    const scalar eta = rndGen_.sample01<scalar>();
     const scalar alpha = 2.0*lambda_/dp;
     const scalar cc = 1.0 + alpha*(1.257 + 0.4*exp(-1.1/alpha));
 
@@ -185,13 +184,13 @@ Foam::forceSuSp Foam::BrownianMotionForce<CloudType>::calcCoupled
         const volScalarField& k = *kPtr_;
         const scalar kc = k[celli];
         const scalar Dp = kb*Tc*cc/(3*mathematical::pi*muc*dp);
-        f = eta/mass*sqrt(2.0*sqr(kc)*sqr(Tc)/(Dp*dt));
+        f = sqrt(2.0*sqr(kc)*sqr(Tc)/(Dp*dt));
     }
     else
     {
         const scalar s0 =
             216*muc*kb*Tc/(sqr(mathematical::pi)*pow5(dp)*sqr(p.rho())*cc);
-        f = eta*sqrt(mathematical::pi*s0/dt);
+        f = mass*sqrt(mathematical::pi*s0/dt);
     }
 
     const scalar sqrt2 = sqrt(2.0);
@@ -199,7 +198,7 @@ Foam::forceSuSp Foam::BrownianMotionForce<CloudType>::calcCoupled
     {
         const scalar x = rndGen_.sample01<scalar>();
         const scalar eta = sqrt2*erfInv(2*x - 1.0);
-        value.Su()[dir] = mass*f*eta;
+        value.Su()[dir] = f*eta;
     }
 
     return value;