Question in the examples/showCases/VofMultiPhase/fallingDroplet

Hi there:
I want give an initial velocity to the drolet, but I don’t know how to do. Should I use the
initializeAtEquilibrium (
lattice, lattice.getBoundingBox(), initializeRhoOnDrop,initializeUOnDrop );

or anything else can I do?
Thank you very much !

SparseBlockStructure3D blockStructure(createRegularDistribution3D(nx, ny, nz));

#ifdef MRT
Dynamics<T,DESCRIPTOR>* dynamics
= new SmagorinskyDynamics<T,DESCRIPTOR>(new VariableOmegaMRTdynamics<T,DESCRIPTOR>(omega), omega, cSmago);

Dynamics<T,DESCRIPTOR>* dynamics2
    = new SmagorinskyDynamics<T,DESCRIPTOR>(new VariableOmegaMRTdynamics<T,DESCRIPTOR>(omega2), omega2, cSmago);

Dynamics<T,DESCRIPTOR>* dynamics
= new SmagorinskyBGKdynamics<T,DESCRIPTOR>(omega, cSmago);

Dynamics<T,DESCRIPTOR>* dynamics2
    = new SmagorinskyBGKdynamics<T,DESCRIPTOR>(omega2, cSmago);


// If surfaceTensionLB is 0, then the surface tension algorithm is deactivated.
// If contactAngle is less than 0, then the contact angle algorithm is deactivated.
TwoPhaseFields3D<T,DESCRIPTOR> fields(blockStructure, dynamics->clone(), dynamics2->clone(), rhoEmpty, densityRatio,
                                      surfaceTensionLB, contactAngle, externalForce, externalForce2,  model);
pcout << "Setting initial fluid flags." << std::endl;

// Initialization

analyticalIniVolumeFraction(fields.volumeFraction, fields.flag, insideFluid, 32);

Box3D bottom  (0,    nx-1, 0,    ny-1, 0,    0);
Box3D top     (0,    nx-1, 0,    ny-1, nz-1, nz-1);

setToConstant(fields.flag, bottom,   (int) twoPhaseFlag::wall);
setToConstant(fields.flag, top,      (int) twoPhaseFlag::wall);

fields.periodicityToggle(0, true);
fields.periodicityToggle(1, true);
fields.periodicityToggle(2, false);


what’s the meaning of this operation: analyticalIniVolumeFraction(fields.volumeFraction, fields.flag, insideFluid, 32);
And I want to initialize the velocity field, but I don’t know how to initilaize the velocity field and what operation to use, where can I find this operation, and what can I do to initialize the velocity field.