var scrollViewer = tree.Template.FindName("_tv_scrollviewer_", tree) as ScrollViewer;
scrollViewer.ScrollToVerticalOffset(20);

<Grid >
<TreeView x:Name="tree" >

    <TreeView.Resources>
    </TreeView.Resources>

    <TreeView.ItemContainerStyle>

        <Style TargetType="{x:Type TreeViewItem}" >
            <Style.Resources>
            </Style.Resources>
        </Style>
    </TreeView.ItemContainerStyle>

    <TreeView.ItemTemplate>

        <HierarchicalDataTemplate   ItemsSource="{Binding data1, ConverterParameter=Name1, Converter={StaticResource MyConverter}}">
            <StackPanel>

                <TextBlock >

                </TextBlock>

            </StackPanel>
        </HierarchicalDataTemplate>
    </TreeView.ItemTemplate>
</TreeView>

for (int i = 0; i < childCount; i++)
{
    var child = VisualTreeHelper.GetChild(this.tree, i);
    if (child.GetValue(NameProperty).ToString() == "Bd")
    {
        var child_Count = VisualTreeHelper.GetChildrenCount(child);
        for (int ii = 0; ii < child_Count; ii++)
        {
            var child_ = VisualTreeHelper.GetChild(child, i);
            if (child_.GetValue(NameProperty).ToString() == "_tv_scrollviewer_")
            {
                var vScrollViewer = (ScrollViewer)child_;

            }
        }
    }
}